错了,应该反一反
(a+b)^3=(a+b)^2*（a+b)
=(a^2+2ab+b^2)(a+b)
=a^3+2a^2b+ab^2+a^2b+2ab^2+b^3
=a^3+3a^2b+3ab^2+b^3
a^3-3a^2b＋3ab^2-b^3＝(a-b)^3